Several processes for the production of ethanol are known. Generally, the production of fuel ethanol involves the fermentation of sugars with yeast. Typically, the sugars are derived from grains, such as corn and wheat. The starches in the grains are subjected to enzymatic hydrolysis in order to produce the sugars, which are then subjected to fermentation to produce ethanol.
Plant materials are a significant source of fermentable sugars, such as glucose that can be transformed into biofuels. However, the sugars in plant materials are contained in long polymeric chains of cellulose and hemicellulose. Utilizing current fermentation processes, it is necessary to break down these polymeric chains into monomeric sugars, prior to the fermenting step.
Recently, processes have been developed for utilizing plant materials, such as corncobs, straw, and sawdust, to produce sugars for ethanol fermentation. Such processes typically comprise pre-treating the feedstock to increase the accessibility of the cellulose to hydrolysis enzymes, and subjecting the cellulose to cellulase enzyme systems to convert the cellulose into glucose.
Methods of converting plant biomass into fermentable sugars are known in the art and in general comprise two main steps: a pre-treatment step to activate the plant structure, and an enzymatic or chemical hydrolysis step to convert the polymeric chains of cellulose and hemicellulose into monomeric sugars. Several approaches have been used for the pre-treatment step, e.g., autohydrolysis, acid hydrolysis, ammonia activation, kraft pulping, organic solvent pulping, hot water pre-treatment, ammonia percolation, lime pre-treatment, caustic soda pulping, or alkali peroxide pre-treatment. Early pre-treatment steps included grinding or milling the feedstock into a powder, which was then mixed with water to form a slurry.
More recently, solvent based pre-treatments, alkali pre-treatments, and acidic pre-treatments have also been described. PCT publication WO/2007/009463 to Holm Christensen describes an alternate pre-treatment, which does not involve the addition of acids, bases, or other chemicals. This pre-treatment process involves soaking the cellulosic material in water, conveying the cellulosic material through a heated and pressurized reactor, and pressing the cellulosic material to produce a fiber fraction and a liquid fraction. During the soaking step, approximately 2.5-3.5 kg of liquid per 1 kg of fiber is added, and is removed again during pressing. The overall pre-treatment process can take about 27 minutes. After pressing, the cellulosic material is exposed to hydrolysis enzymes.
Each pre-treatment technology has a different mechanism of action on the plant structure, inducing either physical and/or chemical modifications. However, the main objective of the pre-treatment is to provide accessibility of the plant material to the enzymes.